ent functions. This fact is
perfectly compatible with numerous beings still retaining simple but
little improved structures, fitted for simple conditions of life; it is
likewise compatible with some forms having retrograded in organization,
by having become at each stage of descent better fitted for new and
degraded habits of life. Lastly, the wonderful law of the long endurance
of allied forms on the same continent--of marsupials [as kangaroos] in
Australia, of edentata [as armadillos, sloths, and anteaters] in
America, and other such cases--is intelligible, for within the same
country the existing and the extinct will be closely allied by descent.

Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatical and geographical changes and to
the many occasional and unknown means of dispersal, then we can
understand, on the theory of descent with modification, most of the
great leading facts in distribution. We can see why there should be so
striking a parallelism in the distribution of organic beings throughout
space, and in their geological succession throughout time; for in both
cases the beings have been connected by the bond of ordinary generation,
and the means of modification have been the same. We see the full
meaning of the wonderful fact, which has struck every traveller, namely,
that on the same continent, under the most diverse conditions, under
heat and cold, on mountain and lowland, on deserts and marshes, most of
the inhabitants within each great class are plainly related; for they
are the descendants of the same progenitors and early colonists. On this
same principle of former migration, combined in most cases with
modification, we can understand by the aid of the Glacial period, the
identity of some few plants and the close alliance of many others, on
the most distant mountains, and in the northern and southern temperate
zones; and likewise the close alliance of some of the inhabitants of the
sea in the northern and southern temperate latitudes, though separated
by the whole inter-tropical ocean. Although two countries may present
physical conditions as closely similar as the same species ever acquire,
we need feel no surprise at their inhabitants being widely different, if
they have been for a long period completely sundered from each other;
for as the relation of organism to organism